The answer to this is determined by the number of days in a woman?s cycle, according to the American Pregnancy Association. If a woman has a 21-day cycle and bleeds for seven days, then she could ovulate and become pregnant.
Ovulation can take place 12 to 16 days before the next period begins, so on a 21-day cycle (from the beginning of one period to the beginning of the next), a woman is fertile during days six through 10 of her cycle. According to the American Pregnancy Association, a woman can become pregnant from intercourse during her period. For a woman who ovulates immediately following her period, conception from intercourse during her period is possible because sperm can live in the body for up to five days.